1. Field of the Invention
The present invention relates to a apparatus and method for increasing the speed of acquisition of data signals modulated onto a high frequency pseudonoise code. More particularly, the present invention relates to a novel range based data acquisition and ranging system for data links employing spread spectrum communication codes.
2. Description of the Prior Art
Ranging systems for data links are well known. Our U.S. Pat. No. 4,8I7,001 explains that ranging systems are classified in U.S. Class 342, subclass 112R and 4512458 and are conveniently subclassified by the method of the coding of the data being transmitted. One class of coding systems employs pseudonoise (PN) spread spectrum codes in the data links which permits data to be modulated onto the PN code transmitted as weak spectral signals which are decoded at the receiver. When the receiver has a PN code generator which is synchronized with the PN code generator in the transmitter, it is possible to recover the data and to also determine the range between the receiver and the transmitter knowing the propagation delay of the PN code.
One of the problems associated with PN transmitters/receiver data links and ranging systems is that once an outage occurs between the receiver and the transmitter, it is necessary to acquire or reacquire the PN code. When the distance between the receiver and the transmitter changes, the PN code of the transmitting station must be searched at the receiver one chip at a time until the receiver PN generated search code locks onto and is synchronized with the signal being received. Heretofore, it was known that the dwell time for the search of a transmitted PN code signal had to be sufficiently long to detect the transmitted PN code at the longest anticipated range between the stations. Since power available for detection is known to vary inversely proportional to the actual to the maximum ranges squared (R.sub.M /R.sub.A).sup.2, it was known that four times the power was available at half the maximum range. However, the dwell or search time was set or predicated upon the weakest signal or longest range from the receiver to the transmitter.
It would be extremely desirable to provide an apparatus and a method for acquiring PN transmitted codes at any predetermined range which is faster than apparatus and methods employed in the prior art data link and ranging systems.